The present invention relates to a system for carrying electrical energy in connections made by means of underground cables.
In particular, it relates to a method of laying cables and a trench filler having a low thermal resistivity, used for laying underground cables for carrying electrical energy.
More particularly, it relates to the use of an inert filler to be disposed in an area around the cables or the parts of the system, such as joints, which are disposed underground.
The flow of current in the cables generates heat, and, if the filler disposed around the cables has a high thermal resistivity, not permitting satisfactory dissipation of the heat, this causes an increase in the temperature of the cables.
The increase in the temperature of the cables beyond the limits tolerated by them causes premature ageing of the cable with possible consequent failures of this cable.
The installation of a system for carrying electrical energy therefore requires the use of fillers having a low thermal resistivity which act in such a way that the cable cannot reach high temperatures and that it is capable of withstanding a higher current load.
In the normal practice of underground cable laying, a trench is excavated in the soil, and a bed of filler, generally consisting of sand, is prepared on the bottom of this trench. The bed of the said material is compacted with mechanical equipment having a vibrating plate. The cables are then laid on this bed. On completion of the laying of the cables, a layer of the same filler is deposited on top of them. Mechanical compaction is carried out on this layer also. The filling of the trench is completed by using inert quarried material, generally extracted on site or in the vicinity, except in those rare cases in which the original ground has a conformation which is particularly favourable for the dissipation of heat, or in which the said original soil has to be restored (as in the case of agricultural soil).
The practice of using inert filling material (sand) as the filler around the cables was adopted in the past particularly to prevent the presence of sharp stones in the original ground from causing damage to the cable sheath. Subsequently, the importance of the heat transmission of the said materials was recognized, and therefore attention was turned towards the search for more suitable fillers, with additional consideration of the stabilization, fluidization and moisture retention of the fillers.
The article by K. Mochlinski, xe2x80x9cAssessment of the influence of soil thermal resistivity on the ratings of distribution cablesxe2x80x9d, Proc. IEE, Vol. 123, No. 1, January 1976, describes the thermal behaviour of different types of ground. In particular, it discloses that sand, which contains a large proportion of materials having the particle size of silt, requires a high percentage of moisture for optimal compaction, greater than that which can be practically provided on site, thus limiting its use.
U.S. Pat. No. 4,177,078 discloses the use of a composition comprising soil, a stabilizing agent which includes a clay mixture, and a dispersing agent.
U.S. Pat. No. 3,719,511 discloses the use of a composition comprising sand, gravel, and large and fine stones, mixed with a cement filler such as Portland cement, and including a lubricant such as fuel oil.
U.S. Pat. Nos. 4,482,271 and 4,925,493 disclose the use of a composition comprising soil and wax.
U.S. Pat. Nos. 4,050,261, 4,062,195 and 4,050,258 disclose the use of a composition comprising Portland cement, aggregates and ash from the combustion of coal (fly ash).
U.S. Pat. No. 4,361,661 discloses the use of a composition comprising sand and an organic adhesive having a filler of inorganic fine particles.
The article by F. Donazzi, E. Occhini and A. Seppi, xe2x80x9cSoil thermal and hydrological characteristics in designing underground cablesxe2x80x9d, Proc. IEE, Vol. 126, No. 6, June 1979, describes the use of selected sands, having an optimized particle size distribution which provides a high density and consequently a small content of voids.
According to the present invention, the Applicant has invented a composition to be used as a trench filler, and having high thermal conductivity, a high degree of moisture retention and also a high thermal stability.
In a first aspect, the present invention relates to a method of laying cables disposed in a trench, comprising the phases of:
removing the original soil in such a way as to prepare the said trench;
disposing a first layer of a first composition in the said trench;
disposing the said cables on top of the said first layer;
disposing a second layer of the said first composition on top of the said cables;
disposing a third layer on top of the said second layer formed by a second composition in such a way as to fill the said trench;
characterized in that the said first composition comprises:
60-90% by weight of crushed sand;
4-15% by weight of silt;
2-12% by weight of cement;
4-15% by weight of water.
In particular, at least one of the said first, second and third layers is compacted mechanically.
Preferably, the said first and the said second layers have a density of more than 1.6 g/cm3 in the dry state.
In a second aspect, the present invention relates to a method of laying cables disposed in a trench, comprising the phases of:
removing the original soil in such a way as to prepare the said trench;
disposing a first layer of a first composition in the said trench;
disposing the said cables on top of the said first layer;
disposing a second layer of the said first composition on top of the said cables;
disposing a third layer on top of the said second layer formed by a second composition in such a way as to fill the said trench;
characterized in that the said first composition comprises:
crushed sand, silt, cement, and water, and has a thermal resistance less than or equal to 0.8 K*m/W.
In a third aspect, the present invention relates to a composition having low thermal resistivity and comprising:
60-90% by weight of crushed sand having a predetermined particle size;
4-15% by weight of silt;
2-12% by weight of cement;
4-15% by weight of water.
Preferably it relates to a composition comprising:
70-80% by weight of crushed sand having a predetermined particle size;
8-12% by weight of silt;
4-8% by weight of cement;
8-12% by weight of water.
Even more preferably it relates to a composition comprising:
75% by weight of crushed sand having a predetermined particle size;
10% by weight of silt;
5% by weight of cement;
10% by weight of water.
This composition is characterized in that the said sand is silica sand.
Preferably, the sand has a particle size in the range from 0.063 mm to 15 mm.
Preferably, the silt has a particle size in the range from 0.002 mm to 0.063 mm.
In a preferred embodiment, the inert sand and silt compound has a particle size distribution in the range from
Yu=xe2x88x920.0649*x6+1.5817*x5xe2x88x9214.403*x4+63.887*x3xe2x88x92148.32*x2+177.94*x+5.1358 
and
Y1=xe2x88x921.9512*x4+13.836*x3xe2x88x9234.44*x2+63.498*x+1.2669 
where
Yu represents the upper limit of the percentage by weight of the material having a size x in mm, and
Y1 represents the lower limit of the percentage by weight of the material having a size x in mm.
The thermal resistance which can be obtained with this composition is less than or equal to 0.8 K*m/W.
Preferably the cement used is of the Pozzolan type.
In a fourth aspect, the present invention relates to a system for carrying electrical energy by means of cables disposed in a trench, the said trench comprising:
a layer of a first composition;
the said cable is embedded in the said first layer;
a layer of a second composition disposed on top of the said first layer,
characterized in that the said first composition comprises:
60-90% by weight of crushed sand having a predetermined particle size;
4-15% by weight of silt;
2-12% by weight of cement;
4-15% by weight of water.